autofill in macro range is constant- how can I code to be a variable range?

You missed part of my post then.

If so, based on your example is that column AR?

Based on your latest post, that is column A (or B or C). So change the code to:

Range("AR4:AW" & Range("A" & Rows.Count).End(xlUp).Row).FillDown

This will find the last used row in column A, and use that to set the filldown limit.